1. Field of the Invention
The field of art to which this invention pertains is the solid bed adsorptive separation of isomeric mixtures. More specifically, the invention relates to a process for separating isomers of diethyltoluenes (DET), and particularly, 2,6- and 3,5-diethyltoluene from other diethyltoluene isomers by employing a solid bed adsorption system.
2. Background Information
Both 2,6- and 3,5-diethyltoluene isomers are important starting materials for making diethyltoluene diamines, from which polyureas and polyurethanes are derived. Also, 2,6- and 3,5-diethyltoluene find application as a desorbent material in certain adsorptive chromatographic separations, e.g., p-xylene from its isomers and p-xylene from mixtures of C.sub.8 and C.sub.9 aromatics.
It is well known in the separation art that certain crystalline aluminosilicates can be used to separate hydrocarbon types from mixtures thereof. Furthermore, X and Y zeolites have been employed in a number of processes to separate individual hydrocarbon isomers. However, no previously published adsorptive chromatographic separation processes have come to light for separating diethyltoluene isomers.
It is, however, known that crystalline aluminosilicates, or zeolites, used in other adsorptive separations of various mixtures, can be in the form of agglomerates having high physical strength and attrition resistance. Methods for forming the crystalline powders into such agglomerates include the addition of an inorganic binder, generally a clay comprising a silicon dioxide and aluminum oxide, to the high purity zeolite powder in wet mixture. The blended clay zeolite mixture is extruded into cylindrical type pellets or formed into beads which are subsequently calcined in order to convert the clay to an amorphous binder of considerable mechanical strength. As binders, clays of the kaolin type, water permeable organic polymers or silica are generally used.
The invention herein can be practiced in fixed or moving adsorbent bed systems, but the preferred system for this separation is a countercurrent simulated moving bed system, such as described in Broughton U.S. Pat. No. 2,985,589, incorporated herein by reference. Cyclic advancement of the input and output streams can be accomplished by a manifolding system, which are also known, e.g., by rotary disc valves shown in U.S. Pat. Nos. 3,040,777 and 3,422,848. Equipment utilizing these principles are familiar, in sizes ranging from pilot plant scale (deRosset U.S. Pat. No. 3,706,812) to commercial scale in flow rates from a few ml per hour to many thousands of gallons per hour.
Also, in some cases illustrated herein, it is necessary to remove components of the feed in three product streams in order to remove undesired components of the feed in an intermediate stream from the extract and raffinate streams. This intermediate stream can be termed a second raffinate stream, as in U.S. Pat. No. 4,313,015 or a second extract stream, as in U.S. Pat. No. 3,723,302, both incorporated herein by reference, the latter incorporating abandoned application Ser. No. 100,105 filed Dec. 21, 1970. This case pertains when a contaminating component in the feed is more strongly adsorbed than the desired product or when two product streams are desired and additional material in the feed can be removed in an intermediate stream. In the latter case, if it is desired to keep the concentration of the contaminating component in the product as low as possible, a first extract is taken off, high in concentration of the desired component and lower in the contaminating product followed by a second extract withdrawn at a point in zone 3 between the desorbent inlet and the first extract point, containing a high concentration of the contaminant and a lower concentration of the desired product. It may not be necessary to use a second desorbent if the desorbent is able to first desorb the lightly held product and then desorb the remaining more strongly held contaminants, as disclosed in the aforementioned abandoned application.
Some separations may require a two-stage process, wherein a first stage separation is operated in the rejective mode to obtain a highly purified raffinate product, e.g., 3,5-DET, and the extract from the first stage is reprocessed in the same or a different column with the same adsorbent/desorbent combination to separate the most strongly adsorbed component, the extract product, e.g., 2,6-DET, from the intermediately-held components of the feed. The separations may also be reversed with the first stage separation operation to obtain a highly purified extract product, e.g., 2,6-DET and contacting a second adsorbent with the first stage raffinate in rejective mode to obtain a highly purified second stage raffinate product, e.g., 3,5-DET. The latter modification is similar to that disclosed in deRosset U.S. Pat. No. 4,213,913 and will be understood therefrom.
The invention may also be practiced in a cocurrent, pulsed batch or continuous process, like those described in U.S. Pat. Nos. 4,159,284 and 4,402,832, respectively. The continuous process described in U.S. Pat. No. 4,402,832 is also capable of operating so as to obtain three product streams as mentioned above.
The functions and properties of adsorbents and desorbents in the chromatographic separation of liquid components are well-known, but for reference thereto, Zinnen et al U.S. Pat. No. 4,642,397 is incorporated herein.
Although numerous uses for isomers of DET or mixtures thereof are known, e.g., as precursors of reactants, e.g., curing agents or isocyanates for making polyurethanes, e.g., diethyltoluene diamine and diethyltoluene diisocyanate, they have recently been found to be a highly advantageous "heavy desorbent" for a chromatographic process for separating para-xylene from mixtures of xylene isomers as disclosed in Zinnen application Ser. No. 197,740, filed May 23, 1988, now U.S. Pat. No. 4,864,069. DET isomers are preferred especially for separating xylene mixtures which also contain C.sub.9 aromatics, the latter of which are difficult to separate from p-diethylbenzene, (p-DEB) a frequently used desorbent in commercial p-xylene separation processes, e.g., the Parex process of the assignee, UOP.
Currently, mixtures of DET isomers are used in the preparation of polyurethane precursors, but it would be highly desirable to make the precursors from highly pure individual isomers of DET in order to obtain higher yields of the desired reactant. Additionally, the yield of individual DET isomers can be increased by isomerizing, at isomerization conditions, the raffinate isomer mixture with an isomerization catalyst selected for a particular isomer, for example, zeolites containing trace metals, as is known in the art, and recycling the raffinate with increased concentration in one of the isomers with the feed to the instant process.